From: Dexuan Cui <decui@microsoft.com>
To: Haiyang Zhang <haiyangz@microsoft.com>,
Stephen Hemminger <sthemmin@microsoft.com>,
"David S. Miller" <davem@davemloft.net>,
"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
KY Srinivasan <kys@microsoft.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Cc: "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>
Subject: RE: Is it safe for a NIC driver to use all the 48 bytes of skb->cb?
Date: Sat, 15 Feb 2020 18:04:13 +0000 [thread overview]
Message-ID: <HK0P153MB0148861FD9AAB88A98084206BF140@HK0P153MB0148.APCP153.PROD.OUTLOOK.COM> (raw)
In-Reply-To: <MN2PR21MB1437345219FA1CC3A75B9875CA140@MN2PR21MB1437.namprd21.prod.outlook.com>
> From: Haiyang Zhang <haiyangz@microsoft.com>
> Sent: Saturday, February 15, 2020 7:20 AM
> To: Dexuan Cui <decui@microsoft.com>; Stephen Hemminger
>
> According to the comments in skbuff.h below, it is the responsibility of the
> owning layer to make a SKB clone, if it wants to keep the data across layers.
> So, every layer can still use all of the 48 bytes.
>
> /*
> * This is the control buffer. It is free to use for every
> * layer. Please put your private variables there. If you
> * want to keep them across layers you have to do a skb_clone()
> * first. This is owned by whoever has the skb queued ATM.
> */
> char cb[48] __aligned(8);
>
> > Now hv_netvsc assumes it can use all of the 48-bytes, though it uses only
> > 20 bytes, but just in case the struct hv_netvsc_packet grows to >32 bytes in
> the
> > future, should we change the BUILD_BUG_ON() in netvsc_start_xmit() to
> > BUILD_BUG_ON(sizeof(struct hv_netvsc_packet) > SKB_SGO_CB_OFFSET); ?
>
> Based on the explanation above, the existing hv_netvsc code is correct.
>
> Thanks,
> - Haiyang
Got it. So if the upper layer saves something in the cb, it must do a skb_clone()
and pass the new skb to hv_netvsc. hv_netvsc is the lowest layer in the network
stack, so it can use all the 48 bytes without calling skb_clone().
BTW, now I happen to have a different question: in netvsc_probe() we have
net->needed_headroom = RNDIS_AND_PPI_SIZE;
I think this means when the network stack (ARP, IP, ICMP, TCP, UDP,etc) passes a
skb to hv_netvsc, the skb's headroom is increased by an extra size of
net->needed_headroom, right? Then in netvsc_xmit(), why do we still need to
call skb_cow_head(skb, RNDIS_AND_PPI_SIZE)? -- this looks unnecessary to me?
PS, what does the "cow" here mean? Copy On Write? It looks skb_cow_head()
just copies the data (if necessary) and it has nothing to do with the
write-protection in the MMU code.
Thanks,
Dexuan
next prev parent reply other threads:[~2020-02-15 18:04 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-15 5:23 Is it safe for a NIC driver to use all the 48 bytes of skb->cb? Dexuan Cui
2020-02-15 15:20 ` Haiyang Zhang
2020-02-15 18:04 ` Dexuan Cui [this message]
2020-02-17 22:31 ` Haiyang Zhang
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